1. Field of the Invention
The present invention relates to an antenna driving device used in a system (smart entry system, tire pressure monitoring system, or the like) equipped with an LF (low frequency) (e.g., 125 kHz) antenna.
2. Description of Related Art
FIG. 3 is a circuit diagram showing an example of a conventional antenna driving device, and FIG. 4 is a waveform diagram showing an example of antenna driving using a reculangular-wave signal.
The conventional antenna driving device 100 has an H-bridge output circuit composed of P-channel MOS field effect transistors 101 and 102 and N-channel MOS field effect transistors 103 and 104, and a positive-phase output signal OUTP and a negative-phase output signal OUTN (e.g., 125 kHz) having mutually opposite phases are fed to the respective terminals of an antenna load 200. At this time, the antenna load 200 having a coil L and a capacitor C is resonance driven at a predetermined resonance frequency f (=1/(2π√LC), where L in the equation is the inductance value of the coil L, and C is the capacitance value of the capacitor C), and a sine-wave output current Io flows through the antenna load 200.
An automobile LF antenna driving device for driving an antenna load using a sine-wave signal is disclosed/proposed in Japanese Laid-open Patent Publication No. 2006-279238 (referred to hereinafter as Patent Document 1) as an example of a conventional technique related to the technique described above.